The Sinking Future of Farmers: An analysis of the complex interplay of agriculture and land subsidence in the Dakahlia Region, Egypt

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1. Introduction

1.1 Study Area

Land subsidence in river deltas is a well-known phenomenon affecting populations in many areas of the world (Evans, 2012). Both human and natural factors that contribute to land subsidence have been identified. An important anthropogenic factor leading to land subsidence is the extraction of groundwater, as will be explained in more detail further in the report (Aly, 2012; Galloway, 2011). Therefore this research report focuses on the effects of groundwater extraction on mutual relationship between agricultural groundwater extraction and land subsidence in the Dakahlia region of the Nile Delta, Egypt.

Figure 1: The Dakahlia Region, indicated in red within the Nile Delta, Egypt (Google Maps, 2015).

Subsidence of the delta has become a topic of major concern to the Egyptian population and government. The land in the Nile Delta subsides with of 2-8 mm/yr combined with a predicted sea level rise of 1.8-5.9 mm/yr (Becker & Sultan, 2009). Especially considering the fact that the Nile Delta is an area of major importance to the Egyptian economy and society. It is commonly referred to as ‘the breadbasket of Egypt’ as its fertile ground accounts for over 93% of Egypt’s agriculture and is home to 50% of the population (FAO, 2015). The fertility of the area is explained by its formation throughout history. The Delta is formed by the division of the branches of the River Nile which spreads out into the Mediterranean Sea. The river branches spread out in a V-shaped fan, starting north of Cairo. In the late Pliocene the Delta started to advance across a marine

embayment by depositing layers of fertile silt, making the deltaic fan expand from east to west and push out into the sea. This happened especially in the Pleistocene through major sea-level changes associated with glacial periods (Hamza, 2009).

This paper focuses in particular on the area of the Dakahlia Governorate in the North-East of Egypt. According to the State Information Service of Egypt, Dakahlia is considered as the base of the Nile Delta and is one of the oldest governorates of Egypt (El-Nahry, 2013). The governorate is home to 6.8% of the population of Egypt and is considered one of the major agricultural governorates. Traditional crops such as cotton, rice, wheat, and maize are produced in the region. Furthermore, the governorate is considered as an area with rich water potentials for aquaculture. The fact that the region is both of high economic importance due to its agriculture and the fact that the region is subsiding, it is particularly interesting as a case study for this research report (Aly, 2012, Becker, 2009).

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1.2 Subsidence in the Nile Delta

As figure 2 illustrates, subsidence of agricultural grounds decreases the relative position of the land to the sea level. As a result of this, saltwater intrudes soils and aquifers, leading to salinization of the agricultural grounds. In 2011, 35% of the irrigated agricultural land in Egypt suffered from salinity (Icarda, 2011). The majority of the salt-affected soils are located in the north central part of the Nile Delta. This leads to a decreased soil quality, affecting food security and the local economy as it can force farmers to scale back their operations (Michel et al., 2010). Figure 2 gives an

overview of the studied system and shows the way in which this problem is self-enforcing.

Figure 2: Diagram of the interplay between agriculture and subsidence

Studying the interplay of land subsidence and the agricultural sector in the Dakahlia region serves two purposes. Firstly, to identify the general understanding of the interplay between subsidence and the agricultural sector and thus to provide policymakers with the necessary information to make decisions that take all consequences into account. As the theoretical framework in chapter 2 will explain, this refers to decisions in the domain of water management and to consequences for both the agricultural sector and land subsidence. The current scientific literature includes many disciplinary studies of aspects of the system studied in this report: For instance, radar

interferometry studies of subsidence rates (Becker & Sultan, 2009) and surveys of farmer attitudes (CLAC, 2009). An integrative view of subsidence and agriculture, however, is missing. Such a view will be useful for policymakers who have to make judgments weighing the benefits and costs to farmers and long-term subsidence rates of e.g. restricting water usage.

Secondly, this paper furthers the quantitative understanding of the expected amount of

subsidence in the future using a mathematical model. A quantitative predictive approach is so far lacking in the literature, but a suitable model developed by Rijniersce (1983) has been found. This model yields unique data that can be used by policymakers in weighing the harms and benefits of policies regarding subsidence and the agricultural sector.

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The problem requires an interdisciplinary approach, since it offers an insight into social as well as earth-scientific consequences and causes of subsidence. Moreover, the problem arises in a complex interactive system that defies being put into a single discipline. If policymakers wish to tackle the problems that are caused by subsidence, they have to be informed about all aspects of the problem and how they relate. For this reason, the disciplines of earth science, human

geography and mathematics are combined. The basic earth-scientific principles of subsidence are qualitatively and quantitatively analyzed while the effects on farmers and their contribution to subsidence fall into the intersection of earth sciences and human geography.

1.3 Structure of the report

To answer the question: “How do land subsidence and agriculture mutually influence each other in the Dakahlia region?” this research paper is divided into four sub-sections. Firstly, it will elaborate on how groundwater extraction for agricultural

purposes contributes to land subsidence. The

focus of this question is on the earth-scientific mechanisms of land subsidence due to groundwater extraction. Secondly, the reversed question is answered: How does subsidence affect the

agricultural sector? In order to answer this question, the physical and geological mechanisms by which subsidence affects the production of agricultural products will be covered. The third research question will answer the question: to what extent is subsidence caused by groundwater extraction and to what extent can it be expected to do so in the future. This is done by the means of a one-dimensional mathematical model of subsidence. Groundwater extraction is only one of the factors influencing land subsidence, but is particularly suitable to be modelled. The wider, societal implications this has on the agricultural sector are identified in the fourth sub-sections. The theoretical framework in chapter 2 functions as a theoretical background of important theories and concepts that are relevant to answer the research questions. In chapter 3, an

overview of the methodology that has been used for the research is presented. Chapter 3 will also discuss the limitations that have been encountered during this research. The answers of the sub-questions are presented in chapter 4. The research paper concludes with a discussion with recommendations for policymakers.

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2. Theoretical Framework

This chapter focuses on the overview of scientific literature regarding subsidence and its

agriculture related causes per discipline, taking into account the social practices and consequences of land subsidence. There are many natural and anthropogenic factors that contribute to

subsidence, but this paper will focus on groundwater extraction as the main cause of land subsidence. Other factors that contribute to subsidence, such as canalization and the building of dams in the Nile River, will be introduced shortly. Because the consequences of subsidence form a risk to food- and water security of the Dakahlia region, these topics are introduced as well. This theoretical framework forms the base of the quantitative model that has been created to calculate the expected amount of subsidence in the region. The combination of these disciplines within the theoretical framework gives a clear overview of the issue of land subsidence and the relationship of this subsidence with agriculture in the Nile Delta region, and thus provides the necessary framework for the final recommendations on agricultural water management for the Dakahlia region.

2.1 Earth-scientific processes

2.1.1 Aquifer systems

To understand the groundwater processes in the soil that lead to subsidence, basic knowledge of aquifer systems is essential. An aquifer is an underground layer of water-bearing material that can be used as a source of groundwater. An aquifer can be either confined or unconfined. A confined aquifer is an aquifer that is saturated with water, but is overlain by a layer of impermeable material. An unconfined aquifer is an aquifer with a water table at atmospheric pressure due to the lack of a confining top layer (Todd, 2005). The Nile Delta aquifer that is discussed in this research report is a confined aquifer system (FAO, 2015). Most aquifer systems have multiple aquifers on top of each other, divided by impermeable aquitards. This research only focuses on the top confined aquifer from which most of the groundwater is extracted (Ayenew, 2011).

Sherif (2001) states that the strata, i.e. sedimentary rock layers, of hydrological importance in the Dakahlia region belong mostly to the Quaternary and Tertiary. More specifically, the geological layer of interest is the Pleistocene top layer of the delta, as this is the groundwater bearing

stratum. During the late Pleistocene, between 35 and 18 thousand years ago, the region of the Nile Delta was an active alluvial plain with widely distributed stream channels. During this geological time the river Nile deposited loose material forming the Delta aquifer. As a result the Nile Delta aquifer was formed, consisting of unconsolidated coarse sands and containing minor isolated lenses of clay. These clay lenses are scattered throughout the soil, which makes it virtually impossible to take these into account for modelling purposes. As the clay layers are only minor features of the aquifer profile, they have not been considered in the one-dimensional model used in this research report. Figure 3 shows the position of the Pleistocene layer relative to layers deposited in other geological periods.

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Figure 3: Cross-section from the Nile Delta from North to South showing Quaternary deposits above thick Tertiary and Mesozoic sequences (Sestini, 1989)

During the Holocene the Delta prograded with 1-7 mm/yr through accretion of sands and clays, resulting in a fluvisol topsoil of 5-17 m acting as a cap above the aquifer (FAO, 2015). Figure 4 represents the soil profile of the top aquifer of the Nile Delta Aquifer system, which according to Ayenew (2011) is the aquifer providing most groundwater. In accordance with the FAO the figure shows how the first 10 meters of soil consist of sandy layer followed by a semipermeable confining clay aquitard. Beneath this layer is the groundwater-bearing stratum consisting of saturated sand. Figure 4 also shows how the Nile Delta sandy aquifer is underlain by an aquitard, restricting the groundwater from leaking to lower lying layers. Therefore, in further modeling it is assumed in this report that the groundwater extraction processes are fully limited to the top aquifer. Note

however that this is a simplified view of reality as other studies, such as Lin et al. (2015), mention that aquifers are in fact interconnected, allowing flow from one aquifer to another. No method to account for this in a simple model could be found and this factor is therefore ignored in the

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calculations.

Figure 4 : Stratigraphic section of the Western Nile Delta near Masrif an Nubariyah (Kahlil, 2014).

This profile is retrieved from a research by Kahlil et al (2014) on the Western Nile Delta, as there was no detailed soil profile of the Dakahlia region available. This also explains the abrupt shift in the base layer in the western section of the profile in figure 4. This shift is due to the fact that the western border of the delta is reached, where different geological processes are in control. It is assumed that this profile is approximately the same as the profile in the Dakahlia region. In accordance with the FAO numbers mentioned earlier, the clay layer above the aquifer is

approximately 5 m thick. However, in addition figure 4 also shows that this clay top layer above the sandy aquifer layer is topped by another surface layer, deposited in a later stadium. According to Kahlil et al. (2014) this layer consists mainly of sands.

The specific assumptions about the soil profile used in the mathematical model will be laid out in section 4.1.2

2.1.2 Geological processes of subsidence

Subsidence due to ground-water withdrawal occurs mostly in unconsolidated sandy sediments that are laid down in alluvial or marine environments such as the Nile Delta. Poland (1984) states that the amount of subsidence affected by compaction is a function of the relative amount pore space in the material as deposited and the thickness of the deposit which is compacted. These sandy delta deposits have a high initial porosity and are therefore vulnerable to consolidation.

Meinzer stated that fluid withdrawal decreases the fluid pressure at the top, causing the covering land mass to consolidate, leading to a lowering surface level (Meinzer, 1925). Consolidation is any process which involves a decrease in water content of saturated soil without replacement of water by air, decreasing the soil volume. Terzaghi (1928) proposed the one-dimensional consolidation theory that made it possible to make quantitative predictions of soil compaction resulting from the drainage of compressible soils. This theory supports the overall theory of subsidence that is caused

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by groundwater extraction. Terzaghi’s theory describes how pore-fluid pressure is transferred to the granular structure of the aquifer when fluid pressure is reduced by lowering of the

ground-water levels. In other words, the increase in effective stress in the confined aquifers is equal to the decrease in fluid pressure. As a result, the total volume of the aquifer is reduced, resulting in lowering of the surface.

The effective stress

is the weight of the soil which is carried by the soil particles. Terzaghi states

that the effective stress or the grain-to-grain load is the outcome of the _{total stres}

s minus the

neutral stress

. The concept of neutral stress

can be described as the pore water pressure

,

which equals the weight carried by water. The forces acting in the aquifer are shown in

figure 5. The effective pressure, also described as the grain to grain load, together with the fluid pore pressure counteract the effective stress.

2.1.3 Sea water intrusion

Seawater intrusion is the process of the infiltration of saltwater from the sea into the hinterland. This process occurs most intensively near the coast because that is the place where freshwater and saltwater aquifers connect. The process is reinforced by land subsidence, sea level rise and

groundwater extraction. The expected sea level rise in Egypt, which is calculated to be about 30 to 80 cm in 2100 (Elsharkawy, 2009), in combination with regional land subsidence increases the pressure of salt water land-inwards. The groundwater levels become relatively lower than the seawater level and therefore the process of saltwater intrusion will increase. Sherif (2001) has shown that in 2001, seawater had already intruded the freshwater aquifer in Dakahlia 63 km land-inwards. Saltwater infiltrates the freshwater aquiferand the aquifer becomes contaminated and salinized. Thus the water quality is decreased, resulting in potential soil salinization.

Figure

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Figure 6 visualizes the situation in Dakahlia, where the saltwater aquifer lies beneath the

freshwater aquifer. Figure 8a shows the intrusion caused by groundwater extraction, and 8b by sea level rise. Figure 8a shows a decline of the freshwater level caused by groundwater pumping in point Q. The red line represents the seawater intrusion after the groundwater extraction. The red line is less steep than the original one before the extraction. This indicates increased pressure of the saline aquifer land-inwards underneath the freshwater aquifer, caused by the decreased groundwater level. Figure 8b shows the intrusion caused by sea-level rise. In contrast with figure 8a, the red line becomes more vertical than the original one before sea-level rise. The seawater has intruded land-inwards; however, the sea has also moved more land-inwards at the surface due to inundation. The pressure of the sea decreases land-inwards when the freshwater aquifers and the lower parts of the Delta are inundated. As a result, the remaining freshwater level is closer to the sea level. This might indicate that sea-level rise causes less seawater intrusion than

groundwater extraction. However, it is expected that humans will do their utmost to prevent flooding; in which case the pressure of the sea will remain, because the height difference between the freshwater level and the sea-level will not change.

2.1.4 Canalization and Damming in the Nile River

Dams are an important source of income for Egypt due to the energy they generate. However, dams have a disrupting nature to ecosystems, since they disturb natural hydrologic processes in rivers by retaining water in reservoirs. This water carries sediment, which remains behind the dams. Before the constructions of dams in the Nile River, the Nile would flood on a regular basis, spreading its sediment over the river banks (Becker, 2009) and thus making agricultural grounds more fertile. However, after the dams have been built, the Nile flow has become more regulated and the areas around the Nile are protected against flooding, and thus the deposition of sediment is blocked by the dams.

In addition to this, there is an abundance of canals in the Nile Delta, as figure 7 shows. These canals enable farmers to effectively use water from the Nile river for irrigation purposes. However,

water diversion by canalization disturbs the natural hydrologic processes and influences the sediment load balance. The abundance of canals in the Nile Delta diverts the Nile River to such an extent that the river no longer floods seasonally, and less sediment is deposited at the delta (Becker, 2009). Consequently, damming and canalization indirectly result in subsidence. When there is no land subsidence or uplift there is a balance in sedimentation and erosion. However due to canalization and damming, sedimentation has significantly decreased, while erosion remains on the same level. Therefore more sediment will get eroded away than gets deposited on the land which results in a lowering of the surface level.

Canalization and damming in the Nile Delta reinforces the process of seawater intrusion in the Nile Delta. The decrease in fluvial activity results in more saltwater infiltration as rivers are less able to refill freshwater aquifers (Elbeih, 2015). Consequently, the pressure of seawater land-inwards will increase and cause more intrusion of saltwater. Figure7shows the extent of canalization in the Nile Delta and illustrates how small the amount of water that reaches the sea is. Only ten of the rivers or canals actually reach the sea.

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Figure 7: An overview of the canalization and water diversion in the Nile Delta. (Stanley, 1998).

2.2 Mathematics

This chapter provides a quantification of groundwater usage and recharge in the Dakahlia region. This quantification enables the use of the model as explained in section 3.2.

2.2.1 Water extraction in the Dakahlia region

The amount of subsidence by the groundwater processes that have been explained in the previous section, depends on the total amount of water that is extracted from an aquifer. Therefore, it is important to look at the water withdrawal in the Dakahlia Region. The total annual water requirement of all socio-economic sectors in Egypt is estimated to be 76 billion m3 _yr-1_(ICARDA,

2011). As figure 8 shows, the agricultural sector is accountable for 86 percent of this water usage. This research paper focuses on water extraction from the Nile Aquifer, which is the source for a large part of the water extraction in Egypt and is situated in the Dakahlia Region (Ayenew et al., 2011).

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Figure 8: Water withdrawal in Egypt (Droogers, 2012)

With 55,5 billion m3 _yr-1 _{most of the agricultural water requirement is supplied by the Nile surface}

water. However, still approximately 19,2 m3 _yr-1 _{comes from groundwater extraction. More}

specifically, according to Mostafa et al. (2004), groundwater extraction for agricultural purposes in the Dakahlia region is estimated at 1.92 billion m3 _yr-1_{in 2000, which has increased from 1.36}

billion m3 _yr-1_{in 1990.} 2.2.2 Water Recharge

The Nile Delta aquifer system is considered a renewable water resource which means that the groundwater extraction is partly compensated by recharge processes. The main recharge source is the infiltration of surplus irrigation water and seepage from the Nile and its branches through the upper clay layer into the aquifer (Elbeih, 2015). Compaction of the aquifer can therefore to a certain extent be restored by this recharge (Milliman & Haq, 1996). According to Sheriff (1999), approximately 25% of the total volume of water that is used for irrigation returns to the

groundwater through agricultural drainage and percolation. According to _{Ayenew (2011),}

these

processes result in an average recharge rate of 400 mm per year in the Nile Delta.

Rainfall over the Nile Delta is rare and occurs mostly during wintertime. The maximum rainfall in the coastal regions is around 180 mm/year but this decreases rapidly in inland regions, down to around 25 mm in Cairo. Therefore rainfall is not considered as an important recharge factor for the Dakahlia region (Geta, 2003).

The Nile Delta Aquifer has a large groundwater capacity, with a rechargeable water storage of 7.5 billion m3_{per year. The total extraction of water in the Nile Delta 2009 has been calculated as 7.0}

billion m3_{. Therefore, for the total Nile Delta the usage can still be considered sustainable in the}

sense that it does not lead to depletion of the groundwater resources yet (Awulachew, 2012). However, the different governorates of the delta have different extraction and recharge rates. Therefore, in the model used in this research report more region-specific numbers of extraction and recharge are used, adding up to a deficit of 18.5 mm per year. The calculation of this number will be presented in chapter 4.

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2.3 Human Geography

In this chapter, the theoretical framework for the Human Geography discipline is presented. First, the influences of subsidence on ecosystem services are described. Then, the theoretical framework behind food security is described.

2.3.1 Influence of subsidence on Ecosystem services in Dakahlia

‘’Ecosystems are dynamic complexes of plants, animals, and microorganism communities that interact as a functional unit’’ (Alcamo, 2003). Humans are an integral part of ecosystems and make use of ecosystem goods and services. Ecosystem services are the benefits that are provided to people by ecosystems, or, as defined in Daily (1997):

“Ecosystem services are the conditions and processes through which natural ecosystems, and the species that make them up, sustain and fulfil human life. They maintain biodiversity and the

production of ecosystem goods”

.

According to Alcamo (2003), ecosystems can function as a bridge between the environment and

human well-being

. People are often dependent on one or more ecosystem services (e.g.

provisioning of food or protection against damage). As shown in figure 9, all types of ecosystem services are linked to several constituents of wellbeing. This suggests that the depletion and degradation of ecosystems can influence human well-being.

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Figure 9: Ecosystem services and constituents of well-being

Four different types of ecosystem services can be distinguished. In this section, all types of ecosystem services are introduced and followed by an indication of the services that are provided by the ecosystem in the Dakahlia region. Finally, the risks of subsidence to ecosystem services are identified.

Provisioning

ecosystem services include products that are provided by ecosystems, such as food,

fuel and fresh water. The ecosystem of the Nile Delta provides water for household uses (e.g. drinking; cooking), industrial uses (e.g. manufacturing, power) and agricultural uses. The Nile River also supplies fish and other goods that are used in aquaculture. _Regulating

ecosystem services

include the benefits that are obtained from the regulation of ecosystem processes, including climate regulation and erosion control. _Supporting

ecosystem services are services that are

necessary for other ecosystem services, such as soil formation or nutrient cycling. The supply of sediment in the Nile Delta enhances the floodplain fertility (Postel, 1997). _{Cultural ecosystem} services are non-material benefits that are obtained from ecosystems through recreation or spiritual enrichment. The ecosystem in the Dakahlia region supplies places for recreational swimming and boating.

As Postel (1997) states, human activities can threaten aquatic ecosystem services that humanity depends on and benefits from. In the Nile Delta, several human activities have been initiated in order to prevent consequences of subsidence, such as relative sea level rise and salinization. One of the causes of subsidence is groundwater extraction. Groundwater is rapidly being extracted from the Nile Delta and as a result of the subsequent saltwater intrusion the groundwater is turning saline (Slootweg, 2008). This strongly influences water quality in the Dakahlia region. Anthropogenic influences on the ecosystem, such as the construction of dams and dikes, pollution, and population/consumption growth have influenced the ecosystem services in the Dakahlia region. The construction of dams, for example, has stopped the supply of sediments and nutrients to the Nile Delta. Concluding, anthropogenic interventions in the Dakahlia region have a large impact on the available ecosystem services, which has a negative impact on the natural environment of the region.

2.3.2 Food Security in the Dakahlia Region

Agricultural practices are highly dependent on the availability of water. If water is scarce, this will have an impact on the food security in a country. The World Food Summit of 1996 has defined food security as: ‘when all people at all times have access to sufficient, safe, nutritious food to maintain a healthy and active life’. It is built on three pillars, which will be introduced in this chapter. The situation in the Dakahlia region will be analyzed according to these three pillars. The first pillar of food security is availability

. This is defined as ‘sufficient quantities of food

available on a consistent basis’. According to Gregory (2005), the availability of food relates to the supply of food through production, distribution and exchange. A variety of factors determine food production, e.g. soil management, crop selection and water availability.

The second pillar of food security is _access

. This is defined as ‘having sufficient resources to obtain

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of food and its consumers. According to the United Nations (1999), the causes of hunger and malnutrition are often not caused by a scarcity of food, but by an inability to access available food, for example due to poverty.

The third pillar of food security is _use

. This is defined as ‘an appropriate use based on knowledge of

basic nutrition and care, as well as adequate water and sanitation’. Various factors influence the quantity and quality of food that reaches its consumer, and the consumer must have enough knowledge to prepare the food so that the food is safe to ingest.

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3. Methodology

3.1 Research design

The design of this research is a descriptive interdisciplinary case study based on the review of scientific literature and the creation of a mathematical model based on quantitative data of the Nile Delta. The research is both quantitative and qualitative, since the research compares geological processes with the experiences of people living in the Nile Delta.

The research question of this paper is “How do land subsidence and agriculture mutually influence each other in the Dakahlia region?”. In order to answer this question, four sub-questions will be answered:

- What are the earth-scientific mechanisms of land subsidence due to groundwater extraction? - How does subsidence affect the agricultural sector?

- To what extent is subsidence caused by groundwater extraction and to what extent can it be expected to do so in the future?

- What are the societal implications of subsidence?

The answers to these questions have been found by several methods. A large variety of scientific literature has been used to further identify the processes that are the causes and consequences of subsidence. A mathematical model has been developed to calculate the expected amount of subsidence in the Dakahlia region in the future. This model will be further explained in chapter 3.2.

3.2 Mathematical model

In the scientific literature, no quantitative models of subsidence in the Nile Delta were found. Therefore a one-dimensional model has been developed, which can be used to predict subsidence as a result of groundwater extraction given certain input parameters. It is based on the model developed by Rijniersce (1983). The model is implemented as a numerical simulation of the rate of subsidence as a result of changes in groundwater levels. While it is explained in greater detail in Mindermann (2015), the key parts of the model are laid out in this section.

Rijniersce himself notes that the model has only empirically been tested in the Netherlands, but states that it can be expected to be applicable in many different settings. An interview with Bouten (2015) has confirmed that the setting in the Dutch delta is similar to the Nile Delta.

The crucial variable to be calculated is the rate of compaction △v. The rate of compaction (or relative compaction) can be calculated for each depth in the ground. It is a percentage, meaning that a value of 0 indicates that no compaction takes place and a value of 1 would indicate that the thickness of a layer is reduced to zero. Since △v can assume different values at each depth, the ground is split into 100 layers and △v is calculated for each. The rate of compaction is then multiplied by the thickness of the layer, which yields the total compaction. The fact that some compaction translates into crack formation rather than subsidence is omitted since, in the region of interest, most compaction happens below 1m depth and cracks only form at the surface (Rijniersce, 1983). The total subsidence is thus equal to the total compaction.

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The rate of compaction △v is calculated as follows: v ln △ = 1 cr p1 p₂ (Equation 1)

where △v _{= rate of compaction}

p₁ = effective stress before increase

p₂ = effective stress after increase

The details on the constant can be found in Mindermann (2015). This report also outlines in_c1

r

detail why the effective stress increases when the groundwater levels decrease. The mechanism is based on the fact that a lower groundwater level entails a lower pore water pressure, which is a force that pushes soil particles apart, thus increasing the soil’s volume.

Given the change in groundwater table and a soil profile that includes the dry and wet density of each layer, the model can be used to calculate the total subsidence.

3.3 Limitations

The studied area is Dakahlia, Egypt. There was no option to visit the region and carry out aspects of the field work. As a result, all the data that is used in this research has been derived from

secondary sources such as the research performed by others. The mathematical model also has some limitations due to this lack of location specific data. Several simplifying assumptions have been made in order to construct the model, such as assuming a simplified soil profile and the estimation of the change of groundwater level in a simplified way. It is possible that under more realistic assumptions the results would differ significantly, although the predicted subsidence falls into the range of expected outcomes. Furthermore, the model has only been tested by Rijniersce (1983) in the Dutch coastal regions.

3.4 Integration of disciplines

This study analyses a complex system that involves both social and natural sciences. As one would expect, these disciplines interface at various instances. The circular relationship depicted in figure 2 is largely responsible for interfaces between earth sciences and human geography and

mathematical techniques are used to quantitatively model earth scientific concepts.

Firstly, there is a connection between earth sciences and human geography regarding the effects of subsidence on the productivity of agricultural land. The Northern Dakahlia region turns out to be mainly affected via the intrusion of salt water, which reduces yields and thus harms farmers. Secondly, the agricultural sector has been identified as a major contributor to land subsidence. The economy in the Dakahlia region heavily relies on agricultural practices which require large amounts of water and thus add to the lowering of the groundwater table and the surface level. These practices are subject to the preferences of local farmers, which is why human geographic research bears on the question of how to change them.

Thirdly, the decrease in groundwater level is taken as an input parameter for the model which combines earth scientific and mathematical knowledge. The other central term from the earth

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sciences is the concept of effective stress. These concepts can be dealt with mathematically due to their associated formulas (e.g. Terzaghi’s formula (1928)).

It is concluded with mathematical methods that groundwater pumping, a factor controlled by social circumstances, significantly contributes to the observed land subsidence via earth scientific mechanisms.

Despite the already numerous intersections of the included disciplines, a more complete view could quantify more aspects of the analysis, thus expanding the reach of mathematics. For instance, neither the impact of policy recommendations on subsidence nor the economic damage due to subsidence could be quantified with the methods in this paper. A further area for

integration between earth sciences and human geography could be an analysis of the favorability of various farming practices based on their social feasibility and impact on subsidence.

4. Results

As the theoretical framework has shown, the implications of subsidence have multiple agriculture-related causes of land subsidence such as groundwater extraction, damming of

upstream parts of the river Nile and canalization in the Nile Delta. In this chapter, the results of the research to the extent of subsidence are presented. The chapter is divided in two subsections, where each subsection focuses on the answer of one sub-question. This contributes to answering the research question, which will be done in the conclusion of this paper.

4.1 Expected amount of subsidence

The first research question focuses on answering the sub-question “To what extent is subsidence caused by groundwater extraction and to what extent can it be expected to do so in the 40 year period of 2015 to 2055?”. This has been calculated by the means of a one-dimensional

mathematical model of subsidence. The simulation code of this model can be found in Appendix II. To quantify the extent to which groundwater extraction is contributing to subsidence in the area of interest, it is necessary to estimate the annual deficit (if any) in groundwater and the soil profile. The deficit lowers the water table in the top aquifer, which leads to subsidence. Combined with a profile of the soil, the model can then calculate the amount of subsidence.

4.1.1 Yearly groundwater deficit

Various assumptions about water usage, recharge and precipitation have to be made to arrive at a value for the yearly groundwater deficit. Total groundwater extraction in the Dakahlia region has been estimated to have grown from 1.36 million l in 1990 to 1.92 million l in 2000 (Mostafa, 2004). Because local data is lacking, uniform pumping over the 3500 km2_{area is assumed which means}

that 55 mm yr-1_{of groundwater is pumped uniformly in 2000. It is assumed that the 2000 level of}

extraction is kept, bearing in mind that according to the trend, it might be higher at present. An approximate 25% of the extracted water re-enters the aquifer by going through the

semi-permeable clay layer (Elbeih, 2015). The rest is lost to evapotranspiration, agriculture and other uses by humans.

Furthermore, it is assumed that pumping occurs only in the top aquifer. As noted previously, the Nile Delta comprises multiple layers of aquifers and aquitards. To simplify calculations, only the upper layer is considered, which is always accessible for pumping.

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The yearly inflow of water to the aquifer is based on recharge from the Nile, precipitation and irrigation water that re-enters the aquifer. The assumed values are displayed in table 1, but require some explanation. As the model works with a yearly recharge deficit, the _potential

recharge from

the Nile of 7.5 billion m3_yr-1_{(Awulachew, 2012) has to be considered. The Nile Delta has a total}

area of 20,000 km2_{and it is assumed that the Dakahlia region receives a share of the Nile’s}

recharge capacity that is proportional to its area. Combining these figures yields a recharge rate of 375 mm yr-1_{from the river. Adding the precipitation of 56mm yr}-1_{in the capital city Mansoura}

(Climate Data, 2012), the total _potential

recharge in the area of interest amounts to 431 mm yr-1_.

Along with an extraction of 412.5 mm this amounts to a yearly groundwater deficit of 18.5 mm.

Groundwater recharge Groundwater extraction

Total potential

recharge from

Nile

7.5 billion m3_yr-1 _{Total extraction}

Dakahlia

1.92 billion m3_yr-1

Area Nile Delta 20,000 km2 _{Area Dakahlia} _{3,500 km}

2

Recharge from Nile 375 mm yr-1 _Extraction _{550 mm}

Annual precipitation Mansoura 56 mm 25% reflow into Aquifer 137.5 mm Recharge 431 mm yr-1 _{Extraction on} balance 412.5 mm

Total annual groundwater deficit: 412.5 mm - 431 mm = 18.5 mm

Table 1: Groundwater recharge, extraction and deficit assumed in Dakahlia region

4.1.2 Soil profile

Various assumptions about the soil profile have to be made in order to apply the model. According to the soil profile in figure 3, a realistic characterization of the four layers of the top aquifer is as follows:

Layer Thickness Material Density (dry)

kg/m3

Density (wet) kg/m3

Initial saturation

Surface 5 Sand 1555 1905 Dry

Clay layer 3 Clay 1600 1760 Dry

Aquifer 30 Sand 1555 1905 saturated

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The figure also shows that the groundwater level, when measured, was approximately at the boundary between the clay layer and the aquifer. The initial level assumed in the model is therefore -8m, which will subsequently decrease due to the groundwater deficit.

The model is applied with these parameters and for simplicity, the subsidence levels found are assumed to be uniform throughout the whole region of Dakahlia. This simplification may not yield accurate results at every point, but it is a necessary restriction of the applied research methods in order to make a hazard map.

The model developed by Rijniersce (1983) had to be adapted in various ways to account for the multi-layered structure of the aquifer system. The corresponding Python code can be found in the appendix.

4.1.3 Predicted amount of subsidence

The simulation has been run with the above soil profile and a groundwater level declining at a rate of 18.5 mm yr-1_{over the 40 year period from 2015 to 2055. A linear decrease in the thickness of}

the aquifer and thus the surface level has been observed. The amount of subsidence is almost constant at approximately 2.5 mm yr-1_{. The total subsidence at the end of the 40 year period was}

10cm. This yields an average annual subsidence of 2.5 mm. The result falls well into the range of 2-8 mm observed by radar interferometry (Becker & Sultan, 2009), which increases confidence in the accuracy of the model. Furthermore, it shows that groundwater pumping is plausibly

responsible for a significant share of the observed subsidence. This result had not been confirmed as of yet. Existing research merely states that lack of sediment, erosion and groundwater pumping make some contribution to subsidence. Without quantitative modelling, the relative contribution of each factor could only be guessed.

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Figure 10: Predicted aquifer thickness from 2015 to 2055.

These results provide a ballpark figure, but should not be interpreted as an accurate prediction since there are many degrees of uncertainty and various simplifying assumptions. The model can, however, be implemented with different assumptions for further research. Given that

groundwater extraction in the Dakahlia region has increased from 1.36 billion m3_yr-1_{in 1990 to}

1.92 billion m3 _yr-1_{in 2000 (Mostafa et al., 2004), it is likely that the amount of extraction will}

further increase with the increasing population and its food demands. Analysing different water use scenarios could be an especially fruitful area for further modelling.

4.2 Influence of subsidence on agricultural practices in Dakahlia

The second sub-question refers to the way in which agriculture is affected by subsidence. In order to answer this question, the physical and social mechanisms by which subsidence affects the production of agricultural products will be covered. The first subsection focuses on the

implications of subsidence on soil-related agricultural processes, whereas the second subsection focuses on the social implications of subsidence in Dakahlia.

4.2.1 Consequences of seawater intrusion and adaptation measures

Groundwater contamination and soil salinization are two of the main consequences of seawater intrusion. These consequences have a major impact on the food and drinking water security, as the groundwater gets contaminated with salt. If the contaminated groundwater is extracted and used for irrigation, it causes salinization and degradation of the soil and withering of crops due to the increased water stress. Besides soil salinization that is caused by groundwater extraction, there is also a natural cause of salinization. Due to the arid climate in the Nile delta, more evaporation (over 1000 mm/year) takes place than precipitation (under 200 mm/year). This higher evaporation is explained by the fact that, more groundwater will get extracted out of the salinized aquifers (ICARDA, 2011). The water evaporates but the salts, if present, are left behind. The entire Nile delta is affected by salinization; 60% of the cultivated lands in the lower delta are affected, 25% of the cultivated area in the middle delta and 20% of the cultivated lands in the upper delta (Kotb, 1999). Figure 11 shows the extent of seawater intrusion in the Dakahlia province.

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Figure 11: Seawater intrusion in Dakahlia (Modified from Stanley, 1998)

The uptake of water by plant roots is based on the principle of osmosis

. Water moves naturally

from an area with lower salinity levels to an area with higher salinity levels due to the osmosis process. Usually, the groundwater has lower salinity levels, which enables the plant to extract water through osmosis. However, if the salinity level of the groundwater are increased to such an extent that it exceeds the salinity level in the plant, the natural process of osmosis cannot take place. This leaves the plant with a water deficit which disables essential plant processes such as photosynthesis (Katerji et al., 2002). Figure 11 shows to which extent the soils have been salinized in the Dakahlia region. The highest soil salinity values can be found near the coast, which is logical because of the presence of salt seawater.

During this research, the subsidence rates that have been calculated using the quantitative model suggest that more regions with currently low to non-saline soils (figure 12) may become more saline in the future. Figure 11 and 12 combined with the elevation of Dakahlia supply a hazard map (figure 13). The areas with brackish water (which is visualized by the red overlay) and low elevation (shown in green), are the most vulnerable to intrusion in the future. The expected subsidence of 10 centimeters, which has been included in the elevation map, combined with an expected sea level rise of about 30 centimeters in 2050 (Elsharkawy et al., 2009), might cause these lower areas to change from an area at risk with brackish water, to a hazardous zone with mostly salt water and therefore more highly and very highly saline soils. Figure 11, from 1998, is over 15 years old. At present, the sea intruded has even further into Dakahlia. The expected subsidence and sea level rise might eventually affect the areas in southern Dakahlia due to increased intrusion. However, their significantly higher elevation (up to 10 m above zero) may prevent this. The hazard map may be useful for farmers and policymakers to adapt to new conditions in advance.

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Figure 12: Salinity levels of the soils in Dakahlia Figure 13: Hazard map (Modified from CLAC, 2009) (Modified from CIAT, 2008)

4.2.2 Identifying the social implications of subsidence

Due to the lack of fresh groundwater that is caused by the intrusion of seawater, farmers in the Dakahlia region are often forced to use alternative water resources, including recycled wastewater with elevated levels of pollutants. As agriculture is by far the largest consumer of water in Egypt (CGIAR 2015), deterioration of the water quality has an enormous impact on the production of food in the so-called ‘breadbasket’ of Egypt. In this subchapter, the view of the farmers on protecting the vulnerability of the region is analyzed.

Roest (1999) has shown that in the region of Dakahlia, water management by farmers is efficiently organized. However, the intrusion of salt water due to groundwater extraction and subsidence is an increasing problem. According to the Central Laboratory for Agricultural Climate (CLAC, 2009), 49% of the inhabitants of Egypt live in the Nile Delta. The area contributes up to 65% of the total national agricultural production. According to the CLAC (2009), the salinity in the Dakahlia region has high-medium values, the water quality is relatively low and the area suffers from a seasonal water shortage. Since the Nile Delta is responsible for over 65% of the food production of Egypt, it is very important to implement measures that don’t have a reducing effect on the quality and quantity of the produced food. The research of the CLAC has shown that farmers are willing to make changes in their agriculture routines to reduce the negative impacts of subsidence.

According to the survey, farmers think that several measures would contribute the most in order to increase the productivity in the region. Unfortunately, the research states that there is only

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limited scientific knowledge available for the farmers. For that reason, scientific literature

regarding the top-three measures that farmers believe to have an impact on the negative effects in the region has been identified.

Firstly, farmers believe that changing cultivars would contribute to stabilize food security in the region. As explained in chapter 4.3, the osmose process within plants is essential for water uptake. The soils in Dakahlia have been salinized due to groundwater extraction and subsidence, and thus soil salinity is a major constraint to agriculture in the region. According to the scientific literature, there are two possible options for this measure. The first option is to incorporate genetic traits via genetic modification that improve the resistance to salinity in a crop (Roy, 2014). Another option is to change the entire species of crops to a type of crop that has a higher salinity tolerance, such as rice. Rice is the most produced crop in the Nile delta, especially in the northern part (Arafat, 2010). The circumstances in the Nile delta should be ideal for rice to grow because rice has a high water requirement, and there is a significant amount of water available in the Nile Delta. Unfortunately the rice yields are under threat due to seawater intrusion caused by subsidence. However, according to Kotb et al. (1999), rice paddies are the best solution to reclaim salinized soils and to make it available again for other crops in the future. There are several salt-resistant rice crops, such as BRRI 23 or 47, that have resulted in higher yields in Bangladesh (Rabbani, Rahman & Mainuddin, 2013) under similar conditions.

The second measure that would be fitting according to farmers, is increasing irrigation

requirements. The CLAC research shows that different combinations of irrigation could improve the capacity of existing irrigation systems in order to overcome the negative impacts of subsidence and climate change. According to the CLAC research, farmers believe that much irrigation is needed in order to wash the salt in the soils of their farmland away. Unfortunately, the

groundwater in the northern Dakahlia region has been salinized in such a high degree that when the soils are irrigated with groundwater, it will only increase the salinization of the soil. At the same time, this extra groundwater extraction will lead to an extra amount of subsidence in the region. Therefore, according to the scientific literature, this measure won’t lead to a reduction of salinization and subsidence in the Nile Region.

Thirdly, farmers believe that changing sowing dates would also contribute to reducing the

vulnerability of the Nile Delta. Crop patterns should be based on Water Usage Efficiency (WUE), in which the water in a plant is compared to the loss of water by transpiration (Bacon, 2004). This could contribute to reduce soil erosion and helps increasing soil fertility and crop yield. According to Huang (2003), implementing a crop rotation scheme of 3-4 years involving combinations of corn, millet and pea significantly improved grain yields and WUE compared to the standard (wheat) monoculture in the dryland region of the Loess Plateau in China. Although this research has been carried out in China, this does show that crop rotation schemes can contribute to a higher crop yield in area’s with a lack of groundwater.

Unfortunately, many constraints for these measures against deterioration of water quality and soil are still relevant, such as the low financial possibilities of farmers. A cost-benefit analysis has shown that the three adaptation measures that are introduced before are not profitable for farmers. According to the CLAC, the only profitable measures would be to cultivate the land for only one season; leave the current cultivated land and move to new land; or to actually leave the

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agri-business and find another career. These three measures are least popular with the farmers according to the CLAC, as each measure can count on the support of 3% of the survey population. Concluding, the use of water-efficient practices and technology is inevitable to reduce vulnerability of the agricultural sector in Dakahlia. The main social implication of subsidence is the increasing risk of food and water scarcity. Farmers wish to fill the lack of fresh water with groundwater, but this leads to an increase of subsidence, and thus to an increase of saltwater intrusion in the region, which leads to a decrease in crop yields. Because of a lack of education, farmers may not always be aware of the implications of the measures against salinization they take. This indicates that the government of Egypt should consider implementing measurements and take care of education of farmers in the Nile Delta. Therefore, in chapter 5 several recommendations for policymakers will be discussed.

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5. Discussion

5.1 Discussion

The main causes and implications of subsidence in the Nile Delta have been analyzed in this report. The various disciplines that have been used to further identify the underlying processes of

subsidence in the Dakahlia region have been combined in order to provide a correct overview of the situation. Since this paper focuses mostly on literature research and the calculation of data from other researches, several improvements to this paper could be made. The choices that were made by the authors of this paper are subjective, since the authors themselves decide what information is relevant and should be included in the paper. _{For further research on the geological} causes and implications of subsidence, it is therefore advised to undertake a field trip to the Dakahlia Region and retrieve the quantitative information regarding water from first-hand research. This could also be combined with gathering local soil profiles to be able to supply the model with local data and predict subsidence in various different locations.

This research report does not go into detail on the effect of damming and canalization. The subsidence as a result of groundwater extraction could possibly be partially compensated by the addition of new sediment to the agricultural fields. This requires a change in water management , decreasing the sediment retaining effects of the dams in the Nile and decreasing the amount of canals in the delta. A more detailed study of the possibilities to counteract the subsidence through water management of the Nile would be a useful addition to this research report.

5.2 Policy recommendations

Although farmers are willing to undertake measures in order to secure their lands and jobs, the CLAC research shows that some of these measures may not be effective enough, and that policy from (local) governments limits progress that farmers are willing to undertake. Research of the International Center for Agricultural Research in the Dry Areas (Icarda, 2011) shows that the developments in the Dakahlia region require corresponding rapid developments in training and capacity building programs. The first recommendation is to invest in education and training for farmers. The research of Icarda (2011) suggests to train the staff of relevant local government technical agencies and farmer technicians to become trainers on natural agricultural resource management, and thus to become responsible for delivering training to farmers at the village level. This training for farmers is essential in order to strengthen adaptive research in the areas of crop water requirements, irrigation schedules, erosion control and the management of salt-affected soils. In order to deal with the predicted increase of salinization it is recommended for farmers to take measures to compensate for the higher salt levels. Changing sowing dates, use of different irrigation methods to improve the capacity of irrigation systems and the use of crops with a higher salt tolerance are all possible methods. For the Dakahlia region it is recommended that farmers use crops with a higher salt tolerance such as the rice cultivars BRRI 23 or 47. The social feasibility of such measures is sometimes questionable, which could be further evaluated.

Water security and (il)legal pumping is another important aspect that policymakers should bear in mind. As the FAO (2015) states, it is hard to measure the amount of groundwater extraction in a region, even if the number of wells is known (the figures cited in this paper are therefore

estimates). This critical note on the quantitative predictions of groundwater extractions in general has to do with the fact that in addition to official drainage of water there is also a significant amount of illegal groundwater pumping carried by individual farmers throughout Egypt. _{In addition}

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to the "official" reuse of drainage water, there is significant "unofficial" irrigation carried out by individual farmers throughout the region. As a result of their various water shortages, they simply place their pump into a nearby field drain and pump drainage water directly onto their field. This "unofficial" drainage water reuse is estimated to be between 2.8 and 4 BCM (Icarda, 2011). Policies to address this issue include stronger law enforcement and economic trading of water usage rights (Chong and Sunding, 2006).

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6. Conclusion

In this paper, the interplay between the agricultural sector and land subsidence in the Dakahlia region has been laid out in an interdisciplinary, holistic manner. In addition, a quantitative model to predict subsidence as a result of groundwater extraction has been developed and applied. Both of these contributions aim to fill gaps in the existing scientific documentation and understanding and provide tools for both policymaking and further research.

This report has shown that there is a complex interplay between the agricultural sector and land subsidence in the Dakahlia region, which is influenced by human as well as natural factors. Groundwater pumping by farmers is the main contributor to a depletion of the upper freshwater aquifer. This depletion results in subsidence through changes in effective stress. The model applied in this study predicts 10 cm of subsidence by 2055 due to groundwater extraction alone. The annual rate of 2.5 mm is in accordance with values observed in the past and provides a new research conclusion: Next to erosion and lack of sediment, groundwater extraction may play a significant role in the observed subsidence rates. Consequently, saltwater intrudes into aquifers and soils, deteriorating the quality of the soil. This has a negative impact on the supporting ecosystem service of fertile ground and groundwater supply. As salinization of the soil and the groundwater impairs crop growth due to increased water stress, the provisioning ecosystem service of food supply is also impaired. This results in potential impacts on the first pillar of food security, namely the availability of sufficient quantities of food on a consistent basis. Therefore, subsidence indirectly influences the agricultural sector, creating a circular causal chain. The increased need for fresh water to flush out the resulting salt water intrusion makes it a

complicated challenge to break this chain. Farmers’ lack of knowledge and willingness to change farming practices further hinders the resolution of the problem.

This study has provided both an integrative view and a quantitative view and shown the

complexity of the self-enforcing mechanisms of subsidence in the Dakahlia region. The results have provided more insight and it should be emphasized that more interdisciplinary research combining quantitative, economic with human geographic methods is recommended to find the most

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Appendix II - Simulation code

import numpy as np class layer: def __init__(self,name,densitywet,densitydry,thickness,porosity): self.name = name self.densitywet =densitywet self.densitydry =densitydry self.thickness =thickness self.porosity =porosity def name(): return self.name def denw(): return self.densitywet def dend(): return self.densitydry def thickness(): return self.thickness g=9.81 #Gravitational constant e=0.6 #Initial pore space

z=-1. #Height relative to surface satd=1905 #Wet soil density

dryd=1555 #Dry soil density watd=1000 #Density of water